The need for subwavelength apertures spans many area of science and technology from subwavelength optical imaging and nanofabrication with near field optics to electrical measurements of cellular biology with subwavelength apertures in micropipettes. The field of this invention is a method for making such a subwavelength aperture at the tip of an optical fiber that also behaves as a lens and for making micropipettes with apertures from as small as tens of nanometers with wall thicknesses as large as 1 mm.
One reason for developing new methods to produce subwavelength apertures is that the field of near-field optics is one of the fastest growing areas of science and technology today. Previously, the most successful method for creating near-field optical apertures was the use of techniques to taper glass in order to provide is a subwavelength region at the tip of the tapered glass structure which could be coated with metal in order to produce a subwavelength aperture at the tip. [A. Harootunian, E. Betzig, M. S. Isaacson, and A. Lewis; Appl. Phys. Lett. 49,674 (1986); A. Lewis, M. Isaacson, E. Betzig, and A. Harootunian, U.S. Pat. No. 4,917,462; Issued: Apr. 17, 1990.]. These methods can be used to either produce a tapered micropipette or can be used to produce a tapered fiber optic element that can then be coated with metal to produce a subwavelength aperture. One aspect of these methods that can be improved is the taper angle of the tip and the core to cladding ratio when an optical fiber is employed. This is necessary for high light transmissions since it is important that the light beam, as it progresses through such a tapered structure, traverses the smallest region of subwavelength dimensions while remaining as much as possible in the core of the fiber. There have been generally two procedures to improve this characteristic of near-field optical elements. One procedure has depended on etching of the fiber tip [Jiang S. Ohsawa, H. Yamada, K. Pangaribuan, T. Chtsu, M. Imai, K. and Ikai. A.; Jpn. J. Appl. Phys. 31,2282 (1992); S. J. Bukofsky and R. D. Grober. Appl. Phys. Lett. 71,2749 (1997)]. A second approach has to been to use an amended versions of the earlier pulling technology [Galina Fish, Sophin Kokotov, Edward Khachatryan, Andrey Ignatov, Rimma Glazer, Anatoly Komissar, Yuri Haifez, Alina Strinkovsky, Aaron Lewis and Kjony Lieberman; Israel Patent Application Serial Number: 120181: Filed: Feb. 9, 1997, PCT filed Feb. 8, 1998]. Micropipettes have not been produced with small apertures and the thick walls that have been achieved in accordance with the present invention.
In the past, there has been no approach to produce a lens with a subwavelength aperture at the tip in an optical fiber or to produce an aperture having a dimension in the region of tens of nanometers or around this region, either bigger or smaller, in a micropipette.
The invention is a method to produce a subwavelength aperture in the tip of a tapered or untapered optical fiber in which the tip also behaves as a lens. The method also allows the production of a micropipette with a small opening in its tip, with very thick walls surrounding the tip.